The compressor sections of axial flow gas turbine engines are subject to particulate erosion due to the ingestion of sand and like matter. Particulate erosion tends to particularly wear away portions of the airfoils which rotate at high speeds. These airfoils are made variously of alloys of titanium, iron and nickel, depending on the temperature which they must endure. The present invention has resulted from extensive work which sought to improve the resistance of airfoils to particulate erosion by imparting a hardened surface to them. In this work, as well as in other work of the prior art, a multiplicity of kinds of coatings have been evaluated, including overlay or deposited coatings which are laid down by plasma spraying, plating, etc. and diffusion coatings wherein elements are diffused into the surface of the article to alter its character.
The approach in the past, as well as that used in making the present invention, has been largely empirical since there is insufficient technology base to allow prediction of erosion behavior on the basis of microstructure or other properties normally measured and used in making material selection. Generally, the object was to provide a hard surface since a general correlation is observed between the hardness of the surface and its resistance to erosion, at least for materials which are potentially useful on metal airfoils operating at 250.degree.-600.degree. C.
The present invention is concerned with the diffusion type coatings, especially those which are comprised of chromium and boron. Generally, borides are known as hard compounds. Therefore, it is logical that boron diffusion into the surface of a structure provides a hard surface, and Hayes in U.S. Pat. No. 3,935,034 discloses boron diffusion into alloys of iron, nickel and cobalt, to provide a wear resistant surface. However, when concentrations of boron are high, there is brittleness at the surface of the material, and it is prone to cracking. Samuel et al in U.S. Pat. No. 3,029,162 discloses the use of a diffusion layer of chromium prior to boron diffusion. While Samuel et al infer the object of their invention is to obtain hardness without brittleness, no relevant data is present beyond hardness measurements. Baranow et al in U.S. Pat. No. 3,622,402 say that the Samuel et al process tends to improve the corrosion resistance of a boronized article, but it is disclosed that the process reduces the mechanical fatigue life of the material by about 50%. Baranow et al further state that simple boronizing of steels also reduces fatigue life by as much as 50%. Their improvement is that the parts are chromized after boronizing, and it is said that this provides at least 80% of the fatigue life which articles had prior to coating, thus providing an improvement over the prior art processes. Thyne et al in U.S. Pat. No. 3,712,798 discloses a method of providing a chromium boride layer on the surface of an article by first depositing an overlay of chromium. That is, instead of interdiffusing the chromium, it must be deposited as a distinct pure layer. Then, boron is diffused into the chromium layer in such a manner that there remains between the boron containing region and the substrate a layer of unadulterated chromium. It is said this provides corrosion resistance and enables a crack-free chromium boride layer on steels where there is a tendency for the layer to be cracked.
As is generally known in the aircraft industry and as mentioned in U.S. Pat. No. 3,622,402, providing a protective surface layer on an article reduces the fatigue strength of the article. This is especially true when the coatings are hard because often associated with the hardness is a low ductility. It is well known that fatigue cracks initiate at the surfaces of articles where the stresses are highest and where there is the greatest propensity for flaws. U.S. Pat. No. 3,779,719 to Clark et al discloses a predominately aluminum coating containing chromium and silicon, which it is said decreases thermal fatigue.
The present invention is particularly concerned with alloys which are used in the higher temperature sections of gas turbine engine compressors. Usually these alloys are called superalloys; other times they are referred to as high temperature alloys, since they have high temperature strength. Compressor parts are particularly prone to mechanical fatigue, which is described in more detail herein. It is well known that putting a coating, such as an electroplate on the surface of a material, will reduce its high cycle fatigue life. Further, boronizing a superalloy will also reduce its fatigue life, as has been reported for other materials. On the other hand, there are some coatings, such as metal-organic coatings, which will not substantially decrease fatigue life but neither will they provide a desired substantial increase in erosion resistance. Consequently, the object of the invention is to provide a coating to a high temperature alloy, which coating does not significantly decrease fatigue life and at the same time which coating substantially increases erosion resistance.